1. Field of the Invention
The present invention relates to digital communications, and more particularly, to methods for correcting carrier frequency and sampling frequency at the transmitter to eliminate the effects of offsets in such frequencies.
2. Description of the Related Art
In a digital communication system composed of at least two transceivers, one serving as a transmitter and the other as a receiver, problems occur if the reference frequencies of the two transceivers are not exactly the same.
There are in general two sources of frequency offsets: carrier frequency offsets and sampling frequency offsets. Carrier frequency offsets result in the received signal being demodulated by a wrong carrier frequency, while sampling frequency offsets result in the data being sampled at the wrong time instants. Typically, the percentages of the carrier frequency offset and the sampling frequency offset would be the same if a single oscillator reference is used to generate the two frequencies at both the transmitter and the receiver. The percentages of the two frequency offsets will be different if different oscillator references are used.
Conventionally, such frequency offsets are only detected and corrected during processing at the receiver end. For example, the receiver can employ a carrier frequency lock loop to determine the carrier frequency offset and a delay lock loop to determine the sampling frequency offset. Such mechanisms are only used to detect the frequency offset between the transmitter and the receiver, and to compensate for offset effects at the receiver end. See, for example: H. Meyr, M. Moeneclaey, and S. Fechtel, Digital Communication Receivers, Wiley-Interscience Publication, 1998; S. Kay, “A fast and accurate single frequency estimator,” IEEE Trans. on Acoustics, Speech, and Signal Processing, December 1989; Viterbi and A. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. on Information Theory, July 1983; M. Fitz, “Further results in the fast estimation of a single frequency,” IEEE Trans. on Communications, February 1994; and D. Messerschmitt, “Frequency detectors for PLL acquisition in timing and carrier recovery,” IEEE Trans. on Information Theory, September 1979.
Conventional techniques for reducing the effects of frequency offsets at the receiving end have many shortcomings. For example, if narrow-band frequency division multiple access (FDMA) is used to provide multiple access from different users, the carrier frequency offsets in the reverse link (from end-users to the base-station) might cause data to overlap in frequency at the base-station receiver. On the other hand, if time division multiple access (TDMA) is used to provide multiple access from different users in the reverse link, sampling frequency offsets might cause data to overlap in time at the base-station receiver.
In more advanced communication systems that employ either multi-user detection in a code division multiple access (CDMA) system, or multi-carrier modulation in an orthogonal frequency division multiplexing (OFDM) system, frequency offsets are particularly damaging in signal detection in the reverse link. For example, in CDMA systems where multiple user access is provided via multiple remote units, multi-user detection can be employed at the base station for interference rejection. See, for example, S. Verdu, Multiuser Detection, Cambridge University Press, 1998. However, the carrier frequency offsets introduced in the reverse link by the various remote units, if not corrected during transmission by the remote units, will destroy the stationary properties of the combined signal as received by the base station, thus greatly degrading the multi-user detection performance. Similarly, in an OFDM system, multiple frequency carriers are used to transmit data to and from multiple users. See, for example, B. Le Floch, M. Alard, and C. Berrow, “Coded Orthogonal Frequency Division Multiplex,” Proceedings of IEEE, pp. 982-996, Vol. 83, No. 6, June 1995. If multiple remote users use different carrier frequencies to transmit data at the same time in an OFDM system, as in the reverse link of a CDMA system, the frequency offsets in both carrier frequency and sampling frequency will cause the data from different users to overlap in both frequency and time, again greatly degrading the multi-carrier detection performance.
Accordingly, there remains a need in the art for techniques for reducing frequency offsets that improve the signal detection capability of the combined signals received from multiple remote units in a base station, and in general between any two transceivers. The present invention fulfills this need, among others.